This invention relates to a method for sealing semiconductor devices and more particularly to a a glass frit sealing technique suitable for the manufacture of highly reliable semiconductor devices.
The sealing of semiconductors is generally classified into (1) resin sealing, (2) metal welding and (3) glass frit sealing. The first method (1) is most desirable for mass production but hardly applicable to the manufacture of semiconductor devices whose interface may be adversely affected by moisture (for example, ones with an offset structure) and semiconductor devices which demand a high degree of reliability, because of moisture permeability of the resin itself. The last two methods (2) and (3) are able to provide a complete hermetic seal and are applicable to the manufacture of the semiconductor devices which are susceptible to moisture and require high reliability although being rather expensive.
Of those said last two methods, the method (3) is more inexpensive and favorable for mass production. However, the last method (3) has the problem of low breakdown voltage as follows: While sealing can be performed with an internal cavity pressure of the order of the atmospheric pressure in the method (2), sealing according to the method (3) is carried out under the condition that the whole of a semiconductor device is held at an elevated temperature of more than 350.degree. C. In the latter method, the internal pressure of a cavity decreases to about 0.4 to 0.5 atmospheric pressure at room temperature after the sealing procedure with the possibility that atmospheric discharge will take place between the bonding wires and the semiconductor device, for example.
Pursuant to Paschen's law, there is a linear relationship between firing voltage V.sub.s (V) and the product of pressure P.sub.o (mm Hg) and the discharge distance d (cm) as indicated in FIG. 1. It is universally known that the firing voltage decreases with pressure except for vacuum sealing procedures. Especially for the semiconductor devices which require high breakdown voltages, this promotes a decrease in yield due to inferior initial pressure resistance and restricts integration density and layout flexibility.
For these reasons, in order to manufacture semiconductor devices with high breakdown voltage, high reliability and high circuit density requirements on an economical basis, it is necessary to increase the firing voltage of atmospheric discharge in the glass frit sealing method. Potential approaches to meet such requirements are as follows:
(1) protection of a metal portion with an electrical insulating coating; and
(2) vaccum sealing.
In connection with the first approach (1), an inorganic insulating coating is desirable since it is to be exposed to a considerably high temperature above 350.degree. C. during sealing. The use of a passivation coating such as SiO.sub.2, Si.sub.3 N.sub.4 and PSG results in defects including unavoidable pinholes occurring during manufacture and cracks caused by the difference in coefficient of thermal expansion between the passivation coating and metals and is insufficient to avoid atmospheric discharge. It is not practical to overlay the surfaces of wires (terminals) and bonding pads with such an insulating coating.
The second approach (2) also has difficulty in establishing a high vacuum seal due to discharge gas from the glass frit during sealing and inferiority in mass productivity and manufacturing cost. Furthermore, in the event that the hermetic seal is broken during use due to fluctuating temperature and oscillation, there is an increased possibility of causing atmospheric discharge.
Another serious problem with the glass frit sealing method (3) is that water may be expelled from the glass frit in its molten state. It is in principle impossible to perform sealing under the condition that the interior of a cavity is deprived completely of water.